Most commercial low density polyethylenes are polymerized in heavy walled autoclaves or tubular reactors at pressures as high as 50,000 psi and temperatures up to 300.degree. C. The molecular structure of high pressure low density polyethylene is highly complex. The permutations in the arrangement of its simple building blocks are essentially infinite. High pressure resins are characterized by an intricate long chain branched molecular architecture. These long chain branches have a dramatic effect on the melt rheology of the resins. High pressure low density polyethylene resins also possess a spectrum of short chain branches generally 1 to 6 carbon atoms in length which control resin crystallinity (density). The frequency distribution of these short chain branches is such that, on the average, most chains possess the same average number of branches. The short chain branching distribution characterizing high pressure low density polyethylene can be considered narrow.
Low density polyethylene can exhibit a multitude of properties. It is flexible and has a good balance of mechanical properties such as tensile strength, impact resistance, burst strength, and tear strength. In addition, it retains its strength down to relatively low temperatures. Certain resins do not embrittle at temperatures as low as -70.degree. C. Low density polyethylene has good chemical resistance, and it is relatively inert to acids, alkalis, and inorganic solutions. It is, however, sensitive to hydrocarbons, halogenated hydrocarbons, and to oils and greases. Low density polyethylene has excellent dielectric strength.
More than 50% of all low density polyethylene is processed into film. This film is primarily utilized in packaging applications such as for meat, produce, frozen food, ice bags, boilable pouches, textile and paper products, rack merchandise, industrial liners, shipping sacks, pallet stretch and shrink wrap. Large quantities of wide heavy gage film are used in construction and agriculture.
Most low density polyethylene film is produced by the tubular blown film extrusion process. Products range from 2 in. diameter or smaller tubes of film to be used as sleeves or pouches, to huge bubbles that provide a lay flat of 20 ft. (when slit along an edge and opened up will measure 40 ft. wide).
Polyethylene can also be produced at low to medium pressures by polymerizing ethylene or copolymerizing ethylene with various alpha-olefins using heterogeneous catalysts based on transition metal compounds of variable valence. These resins generally possess little, if any, long chain branching and the only branching to speak of is short chain branching. Branch length is controlled by comonomer type. Branch frequency is controlled by the concentration of comonomer(s) used during copolymerization. Branch frequency distribution is influenced by the nature of the transition metal catalyst used during the copolymerization process. The short chain branching distribution characterizing transition metal catalyzed low density polyethylene can be very broad.
U.S. patent application Ser. No. 892,325 filed Mar. 3, 1978, and refiled as Ser. No. 014,414 on Feb. 27, 1979 in the names of F. J. Karol et al and entitled Preparation of Ethylene Copolymers In Fluid Bed Reactor, discloses that ethylene copolymers, having a density of 0.91 to 0.96, a melt flow ratio of .gtoreq.22 to .ltoreq.32 and a relatively low residual catalyst content can be produced in granular form, at relatively high productivities if the monomer(s) are polymerized in a gas phase process with a specific high activity Mg-Ti containing complex catalyst which is blended with an inert carrier material.
U.S. patent application Ser. No. 892,322 filed Mar. 31, 1978, and refiled as Ser. No. 012,720 on Feb. 16, 1979 in the names of G. L. Goeke et al and entitled Impregnated Polymerization Catalyst, Process For Preparing, and Use For Ethylene Copolymerization discloses that ethylene copolymers, having a density of 0.91 to 0.96, a melt flow ratio of .gtoreq.22 to .ltoreq.32 and a relatively low residual catalyst content can be produced in granular form, at relatively high productivities, if the monomer(s) are polymerized in a gas phase process with a specific high-activity Mg-Ti-containing complex catalyst which is impregnated in a porous inert carrier material.
U.S. patent application Ser. No. 892,037 filed Mar. 31, 1978 and refiled as Ser. No. 014,412 on Feb. 27, 1979 in the names of B. E. Wagner et al and entitled Polymerization Catalyst, Process for Preparing And Use For Ethylene Homopolymerization discloses that ethylene homopolymers having a density of about .gtoreq.0.958 to .ltoreq.0.972 and a melt flow ratio of about .gtoreq.22 to about .ltoreq.32 which have a relatively low residual catalyst residue can be produced at relatively high productivities for commercial purposes by a low pressure gas phase process if the ethylene is homopolymerized in the presence of a high-activity Mg-Ti-containing complex catalyst which is blended with an inert carrier material. The granular polymers thus produced are useful for a variety of end-use applications.
The polymers as produced, for example, by the processes of said applications using the Mg-Ti containing complex catalyst possess a narrow molecular weight distribution, Mw/Mn, of about .gtoreq.2.7 to .ltoreq.3.6, and preferably, of about .gtoreq.2.8 to .ltoreq.3.4.